Description and History
Coal, which is deposits of carbon, has been known since antiquity. Coal is a dark rock of varying hardness, formed from plant remains buried in sediment. Much of this coal formed during the plant-rich Carboniferous Periods of around 300 million years ago. Although quite abundant, the coal being mined now will eventually be depleted.
Coal is burned as a fossil fuel. Although some coal was no doubt recovered and burned in prehistoric times, wood is more convenient as a low-temperature fuel. Coal only became necessary for Iron Age furnaces that burn much hotter. Formal surface mining of the mineral picked up during medieval times as ironworks increased substantially.
Large-scale coal extraction is a signature of the Industrial Revolution starting in the 18th Century. Coal fueled boilers for the steam engine; soot from coal fires blackened England; coal miners now working underground organized into unions. The steam engine made deep underground mining possible by pumping out water and pumping in air. Consequently, many of the negative aspects of the Industrial Revolution relate to coal mining, including exploitive work conditions, lung diseases, mine collapses, and fires.
Coal is held in mixed regard in modern politics and society. On the one hand, coal is the most abundant fossil fuel and therefore a source of future energy and jobs. On the other hand, coal is considered dirty, an irredeemable polluter, and a source of greenhouse gas (see below). Expect much more discussion and activity about coal.
Mining and Production
Great quantities of coal are mined in comparison to any other mineral. From the US Geologic Survey report for 2010:
In other words, the dollar value of coal by itself was about equal to all the metal mining or half of all the mineral mining. The major coal-producing countries are China (40%) and the United States (16%).
Coal mining is the canonical example of strip mining, even to the point of leveling and eliminating hills. Surface mines often cover several square miles. In modern practice, the overburden (rock other than coal) is set aside during mining, then replaced in such a way as to minimize erosion.
Purely underground mines become, in some cases, vast caverns interrupted by occasional support pillars. Underground mining is heavily automated these days, often with remotely controlled equipment, which minimizes the danger.
Extracted coal is usually washed and crushed to a powder before being shipped to its destination, often a smelter or furnace. The coal may also be treated to remove noxious pollutants such as sulfur and mercury; otherwise, these elements must be scrubbed from the products of burning.
Coke is coal treated by baking to drive off volatile contaminants. Coke is almost pure carbon; it is introduced into iron smelting to create steel.
Properties and Uses
Coal is a black mineral rock found in a range of hardnesses. The hardest coal is referred to as anthracite in the United States and is used for residential and commercial purposes. Softer coal, bituminous, is used for industrial and manufacturing purposes. There is also a range of softer rocks (lignite) from bituminous all the way to peat. Peat is compressed plant matter formed in swamps, not yet a mineral.
Most coal is used as a fuel, either for space heating or for driving steam turbines for electricity. Coal is the world's biggest (40%) source of electricity.
Coal gasification turns coal into a fuel gas by reacting it with oxygen and water to produce a mixture of carbon monoxide and hydrogen. This synthetic fuel mix can be burned for power or it can be further converted into gasoline or diesel fuel.
A relatively small amount of coal is used in the chemicals industry. Coal is essentially the element carbon and can be converted to methane or coal tar (a mixture of organic molecules). Coal tar is the starting point for many synthetic dyes. Methane is a starting point for much organic chemistry.